SLS In the mid-1950's, US Air Force-funded studies identified the optimum long-term solution for space launch. The studies indicated the desirability of segmented solids for a first stage to achieve low cost, high reliability and flexibility of basic booster size by adding or subtracting segments. Studies further showed that oxygen-hydrogen propellants, with their very high specific impulse, were a preferred choice for upper stages, where mass was more important. This choice also resulted in minimum systems cost. More...